The LG Thrive is being ignored by both LG and AT&T. When I last talked to LG about this they said AT&T has the code for the Thrive for 2.3. AT&T said they were doing a "slow rollout". After that, they recanted their comments on posts on the various forums, including Official LG Phone Forum
and AT&T's own forum (which has turned into a gripe forum with no resolutions).
LG seemed to get rather snooty about their post saying that it wasn't up to them to get the software out, it is up to AT&T. Since then, AT&T has released another Go Phone, which is Android based. I was told by the local AT&T guys to go buy the other phone "since they are so cheap". (Yeah $150 is cheap to someone who makes less than $250 a week...sure)
IC 4.x will not hit the Thrive. Too little memory and other storage issues. Along with its fixed 600Mhz CPU and you get something that will crawl worse than an 3.6 Ghz netbook loaded with Windows 7 (or Ubuntu 11.x/12.x for that matter - Unity sucks....)
There are some very nice phones you can get as upgrades for a great promotional deal. I just got two and working on getting a third. While I like the Thrive, and my family has several, they have simply become a nice toy for wifi now.
Save your money. Find something on eBay or Craigslist.org. That or try one of the various ROMS floating about for the P506 (Thrive).